Technically, it's a sprite. A single image used to round all four corners of a div block. The idea was inspired by this ALA article.

Previously, I've used the clever nifty corners javascript package. It's more elegant on the development side. But in operation, it's a bit clunky and results in a noticeable bounce as the tweaks are applied to the output.

Css-defined background images fill in more smoothly, but require additional sweat in putting together the page. First, you must create the appropriate image(s) (the background and foreground colors must match the css settings.) You only need one 16x16 round image like this: